Thermoplastic elastomers (TPE) are both elastomeric and thermoplastic. They are distinguished from thermoset rubbers which are elastomeric but not thermoplastic due to the cross-linking or vulcanization of the rubber, and are distinguished from general thermoplastics which are generally stiff and hard, but not elastomeric.
Thermoplastic vulcanizates (TPVs) are a class of TPE in which cross-linked rubber forms a dispersed, particulate, elastomeric phase within a thermoplastic phase of a stiff thermoplastic such that TPE properties are achieved. TPVs or TPV compositions are conventionally produced by dynamic vulcanization. Dynamic vulcanization is a process whereby a rubber component is crosslinked, or vulcanized, under intensive shear and mixing conditions within a blend of at least one non-vulcanizing thermoplastic polymer component at or above the melting point of that thermoplastic. Typically, the rubber component forms cross-linked, elastomeric particles dispersed uniformly in the thermoplastic. See, for example, U.S. Pat. Nos. 4,130,535; 4,311,268; 4,594,390; and 6,147,160. Dynamically vulcanized thermoplastic elastomers consequently have a combination of both thermoplastic and elastic properties. Conventional plastic processing equipment can extrude, inject, or otherwise mold, and thus press and shape TPV compositions into useful products alone or in composite structures with other materials.
TPEs and TPVs can be foamed by incorporating a modifier or filler or other components. See, for instance, WIPO Publications WO2004/016679 and WO2007/0044123. WO 2013/062685 suggests the use of thermo-expandable microspheres in creating foamed TPV compositions, asserting that such foamed compositions will have homogeneous closed cell structures, and may therefore exhibit superior relaxation performance, low water absorption, and desirably soft touch. Other references of potential interest along these and similar lines include: WIPO Publication WO2015/095987; U.S. Pat. Nos. 5,728,744, 6,268,438, 6,288,171, 6,399,710, 6,720,361, 6,713,520, 7,294,675, 7,319,121, 7,326,471, 7,652,097, 7,829,623, 7,964,672, and 8,178,625; US patent publications US2010/0119855, US2013/0101826, US2013/072584; Japanese patents JP5972200B, JP4660016B, JP3964669B, JP5481278B; Japanese Patent Publication No. 2005-088718; Chinese patent publication CN104072881A; Korean patent publication KR20160083324A; and Korean patent KR101013827B.
However, using thermo-expandable microspheres in foaming TPVs presents some challenges that have not yet been adequately addressed. These microspheres are typically very small, dust-like particles, which can be very difficult to introduce to an extruder or other like apparatus for foaming a TPV composition with the microspheres. As such, the microspheres are frequently provided in the form of a masterbatch comprising microspheres dispersed in carrier polymer, which makes for significantly easier handling and introduction to the foaming/blending process. However, using such a masterbatch creates its own difficulties. Relatively small amounts of microsphere are typically blended with TPVs to make the foamed TPV, which makes it difficult to achieve homogeneous distribution of the microspheres within the TPV. This in turn leads to non-uniform properties, for instance, non-uniform specific gravity. This non-uniformity may be exhibited, for instance, along the profile of an article or other material formed form the foamed TPV, and/or among articles, TPV pellets, or the like produced at different times during a production run in a commercial-scale manufacturing facility.
Therefore, it would be highly desirable to find a suitable means of increasing dispersion of microsphere masterbatch in a TPV composition during blending/foaming.